The dumb money indicators are typically made up of retail traders and trend-followers. This is NOT to say that all (or even most) retail mom-and-pop investors, and certainly not most trend-followers, are "dumb". In fact, they are by definition correct during the bulk of a trend.
The smart money indicators are mostly made up of institutional accounts. These traders are often hedging day-to-day moves in the market, and therefore are often trading against the prevailing trend. Again, it is only when these traders move to an extreme that a market is most likely to reverse in their direction.